This invention was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties.
1. Field of the Invention
The present invention relates to the manufacture of electro-optical devices, and in particular, to a method for manufacturing layered optical devices by transferring a thin electro-optical material layer from a transfer substrate optimized for electro-optical material layer formation, to a handle substrate such as a circuit-containing substrate, an insulting substrate, or optically transparent substrate.
2. Background of the Invention
Electro-optical devices include electro-optical waveguides, electro-optical modulators, optical switches, optical amplifiers, semiconductor optical amplifiers, waveguide lasers, waveguide optical amplifiers, waveguide second harmonic generators, surface normal spatial light modulators, optical parametric oscillators, and high frequency Bragg cells. These electro-optical devices include an electro-optical layer composed of an electro-optical material such as a photorefractive material and a ferroelectric crystal material. Examples of these electro-optical materials include LiNbO3, BaTiO3, TeO2, LiTaO3, KNbO3, BaSrNbO3, GaAs, InGaAsP, GaAs/AlGaAs multiquantum wells, InAsP/InGaP multiple quantum wells, InGaAIAs/InAIAs multiple quantum wells, and PLZT. All of these materials provide advantageous materials for electro optical components such are optical waveguides, electro-optical modulators, and optical switches due to their high electro-optical and photorefractive properties.
Rare-earth doped LiNbO3 and BaTiO3 are advantageous materials for optical amplifiers and waveguide lasers. Thin layers of PLZT are advantageous for use in spatial light modulators when the layers are illuminated perpendicular to the PLZT surface. Other materials include KNbO3 which is advantageous for second harmonic generation (SHG) due to its large non-linear optical effect. AgGaSe2 is advantageous for use in optical parametric oscillators (OPO).
In conventional electro-optical devices, the electro-optical layer is manufactured by growing a thin film layer of the electro-optical material on a substrate or the electro-optical layer is formed in a bulk substrate that has electro-optical properties. To obtain a high quality thin film electro-optical layer, the layer is typically grown on the substrate at a growth or annealing temperature of 500xc2x0 C. to 1000xc2x0 C. and is often grown on a single-crystal substrate in order to obtain a highly oriented or single crystal thin film electro-optical layer. The high growth temperature can prevent the utilization of electro-optical layers overtop of or on certain substrates. For example, it would be desirable to grow electro-optical material and a substrate that contains CMOS integrated circuits. Typically, such a circuit can withstand temperatures up to 400xc2x0 C. to 500xc2x0 C. This is especially relevant when the CMOS integrated circuit includes metal interconnections between circuitry layers. In addition, it is desirable to be able to grow the electro-optical material directly overtop of CMOS circuitry and use the CMOS circuitry to control the electro-optical component. The material layers that reside overtop of the CMOS circuitry are typically amorphous silicon oxide or silicon nitride layers and thus is not suitable as a substrate for the growth of highly oriented thin film electro-material layer. Therefore, it is generally not possible to obtain the best quality thin film electro-optical layer by growing the electro-optical layer directly on a CMOS substrate.
It is often desirable that the electro-optical components be fabricated using electro-optic material that is single-crystal to achieve the highest performance. Single crystal electro-optic material is typically obtained by growing crystals in bulk form or growing epitaxial layers on a bulk crystal.
A disadvantage with current electro-optical devices is that the electro-optical layer is formed on a substrate under conditions, which may not be optimal for electro-optical layer growth. Consequently, the quality of electro-optical layer may be limited due to its non-optimal growth condition.
Alternatively, an electro-optical layer may be grown on a substrate not optimal for an electro-optical device function. For example, a conventional optical modulators utilize coplanar electrode structure on the surface of the electro-optical material have a relatively high driving voltage to produce the necessary electric fields to modulate the electro-optic material. It would be desirable to have the metal electrodes defined above and below the electro-optical material with small distance separation to the electro-optic material to achieve high electric fields and thus requiring low driving voltages that allow higher modulation frequencies during the operation of the optical modulator and optical switches. In addition, to achieve high operating frequencies, it is desirable to arrange the metal electrodes in a transmission line arrangement on an insulating substrate with optimal dielectric constant such as quartz, semi-insulating GaAs, glass, polymer, sapphire or other suitable materials to be able to implant high frequency, low loss transmission lines.
As a result, the electro-optical device formed may be limited due to the non-optimal growth substrate characteristics.
In accordance with the present invention, an electro-optical device manufacturing method is provided in which an electro-optical layer is transferred from a growth substrate to a handle substrate. The method includes using wafer bonding and hydrogen ion implant splitting to transfer the electro-optical layer. The handle substrate may include CMOS or GaAs circuitry. The resulting handle substrate with transferred electro-optical layer may be utilized to form various optical devices.
According to one aspect of the present invention, a method is provided for manufacturing an electro-optical device comprising the steps of providing a transfer substrate including a growth substrate and an electro-optical layer and implanting hydrogen ions or hydrogen ions in combination with other ions such as helium or boron into the transfer substrate to form an intermediate hydrogen ion implant layer thereby defining an electro-optical layer portion of the transfer substrate which includes at least a portion of the electro-optical layer. A handle substrate is then provided and bonded to the transfer substrate along a bonding surface to form a joined structure. The joined structure is heated to a temperature sufficient to split the joined structure along the hydrogen ion implant layer thereby to transfer the electro-optical layer portion of the transfer substrate to the handle substrate and to form a splitting surface on the electro-optical layer portion. In alternate embodiments, the handle substrate comprises CMOS or GaAs circuitry. In further alternative embodiments, the hydrogen ion implant layer forms within the electro-optical layer or within the growth substrate layer.
According to another aspect of the present invention, a method is provided for manufacturing an electro-optical device comprising the steps of providing a growth substrate layer having an electro-optical layer and implanting hydrogen ions in the electro-optical layer to form an intermediate hydrogen ion implant layer thereby defining an electro-optical layer portion of the electro-optical layer. A first index-of-refraction material is deposited on the electro-optical layer. A first metal electrode is formed on the first low index-of-refraction material. A dielectric layer is deposited and planarized to form a planarized dielectric surface over the first metal electrode. The planarized dielectric surface is bonded to a handle substrate to form a joined structure. The joined structure is heated to a temperature sufficient to split the hydrogen ion implant layer thereby to transfer the electro-optical layer portion.
According to yet another aspect of the present invention, a method is provided for manufacturing an electro-optical device comprising the steps of providing a transfer substrate having a growth substrate layer. Hydrogen ions are implanted in the growth substrate to form an intermediate hydrogen ion implant layer thereby defining a growth layer portion of the transfer substrate which includes at least a portion of the growth substrate layer. A handle substrate is provided having a planarized surface of either a GaAs circuit or a CMOS circuit. The growth substrate layer is bonded to the planarized surface to form a joined structure. The joined structure is heated to a temperature sufficient to split the joined structure along the hydrogen ion implant layer thereby to transfer the growth layer portion of the transfer substrate to the handle substrate and to form a splitting surface on the growth layer portion. A thin film electro-optical layer is grown on the splitting surface.
According to another aspect of the present invention, an electro-optical device is provided which includes a substrate having either CMOS or GaAs circuitry formed therein. An electro-optical layer is formed over the substrate.
A feature of the present invention relates to growing an electro-optical layer on an optimized growth substrate, which is different from an optimized handle substrate and may contain GaAs or CMOS circuitry.
An advantage of the present invention relates to growing electro-optical layers with enhanced electro-optical properties resulting from growing the electro-optical layers on an optimized growth substrate and under optimal electro-optical layer growth conditions.
An additional feature of the present invention relates to locating an electro-optical layer directly over GaAs or CMOS circuitry with the GaAs or CMOS circuitry controlling the optical properties of the electro-optical layer. For example, the present invention may be employed in a cross-point optical routing array, which has optical switches in the form of electro-optical layers formed directly over GaAs or CMOS circuitry with the GaAs or CMOS circuitry controlling the switching. As a result, the GaAs or CMOS circuitry can provide cross-point switch for optical routing of fiber-optic signals.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.